Estrogens have multiple effects on cell function and survival in the central nervous system. The proposed research explores the sites and mechanisms through which estrogens control gonadotropin-releasing hormone (GnRH) neurons. These neurons form the final common pathway for central regulation of fertility; their proper function is thus critical to survival of vertebrate species. Depending on level and duration of exposure, estrogen can inhibit (negative feedback) or stimulate (positive feedback) GnRH release. The female reproductive cycle is characterized by a natural switch between negative and positive feedback that is critical to initiating the ovulatory process. How negative feedback, positive feedback and the switch between these modes are brought about is unknown. This physiological phenomenon combined with the relative simplicity of their inputs and the known effects of estrogen on GnRH secretory output make GnRH neurons an intriguing model for studying how multiple central estrogen actions are integrated to produce a response. Electrophysiological, molecular and cellular biological approaches will be used to study living GnRH neurons tagged with green fluorescent protein. Estrogen effects will be studied in physiological models of negative and positive feedback, during the estrous cycle and after acute exposure to the steroid. The working hypothesis is that estrogen feeds back to regulate GnRH release at two sites: transsynaptic via estrogen-sensitive afferents and directly on GnRH neurons; at each site, two types of signaling mechanisms are activated: genomic and non-genomic. Aim 1 will determine how estrogen acts to affect ionotropic synaptic inputs to GnRH neurons. Aim 2 will examine estrogen- induced changes in intrinsic conductances, excitability and firing pattern of GnRH neurons. Aim 3 will study the signaling mechanisms activated in GnRH neurons by estrogens, exploring estrogen-induced changes in the expression of genes coding for proteins that affect cell excitability in GnRH neurons, as well as non-genomic actions mediated by kinase cascades initiated by estrogen action. Data obtained from experiments in these three Aims will expand our understanding of how estrogen actions via different mechanisms and at different sites are integrated to impact reproduction by revealing mechanistic differences that underlie positive and negative feedback regulation of GnRH neurons. Knowledge gained from these studies will foster development of novel rational strategies for fertility manipulation in humans, domestic animals, and endangered wild species, and may be applicable to understanding estrogen action on more complex neural systems, such as hippocampus and cortex.